| Acromion |
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| Left scapula. Posterior view. Acromion shown in red. |
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| Latin |
Acromion |
| Gray's |
subject #50 203 |
In human anatomy, the acromion (from Greek: akros, "highest", ōmos, "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The acromion is a continuation of the scapular spine, and hooks over anteriorly. It articulates with the clavicle (collar bone) to form the acromioclavicular joint.
Contents
- 1 Human anatomy
- 1.1 Surfaces
- 1.2 Borders
- 1.3 Variation
- 2 In other animals
- 3 Additional images
- 4 Notes
- 5 References
- 6 External links
Human anatomy[edit]
The acromion forms the summit of the shoulder, and is a large, somewhat triangular or oblong process, flattened from behind forward, projecting at first lateralward, and then curving forward and upward, so as to overhang the glenoid cavity.[1]
Surfaces[edit]
Its superior surface, directed upward, backward, and lateralward, is convex, rough, and gives attachment to some fibers of the deltoideus, and in the rest of its extent is subcutaneous. Its inferior surface is smooth and concave.[1]
Borders[edit]
Its lateral border is thick and irregular, and presents three or four tubercles for the tendinous origins of the deltoid. Its medial border, shorter than the lateral, is concave, gives attachment to a portion of the trapezius, and presents about its center a small oval surface for articulation with the acromial end of the clavicle.[1]
Variation[edit]
There are three morphologically distinct types of acromia and a correlation between these morphologies and rotator cuff tear:[2]
| Type |
Prevalence |
Angle of
anterior slope |
Rotator
cuff tear |
| Flat |
17.1% |
13.18 |
3.0% |
| Curved |
42.9% |
29.98 |
24.2% |
| Hooked |
39.3% |
26.98 |
69.8% |
Os acromiale[edit]
The acromion has four ossification centers called (from tip to base) pre-acromion, meso-acromion, meta-acromion, and basi-acromion. In most cases, the first three fuse at 15–18 years, whereas the base part fuses to the scapular spine at 12 years. However, in between 1% and 15% of cases, this osseous union fails and the acromion remains a separate bone. This condition is referred to as os acromiale, but rarely causes pain. Earlier estimates of its prevalence were as low as 1.4%, and this higher estimate was made by Sammarco in the year 2000, based on radiographic and anatomical studies.[3][4]
Four types of os acromiale can be distinguished:[5]
- A non-union between the meso- and meta-acromia, the most common or typical os acromiale
- A non-union between the pre- and meso-acromia
- A non-union between the pre- and meso-acromia; and between the meso- and meta-acromia, atypical
- A non-union between the pre- and meso-acromia; between the pre- and meso-acromia; and between the meta- and basi-acromia
This feature was common in skeletons recovered from the Mary Rose shipwreck: it is thought that in those men, much archery practice from childhood on with the mediaeval war bow (which needs a pull three times as strong as the modern standard Olympic bow) pulled at the acromion so much that it prevented bony fusion of the acromion with the scapula.
Although historically regarded as an incidental finding, the os acromiale may occasionally produce symptoms from subacromial impingement or instability at the site of non-union.[6] Dynamic US evaluation may demonstrate hypermobility of the non-union during shoulder motion or graded compression with the probe in symptomatic individuals.[7]
See also: Ossification of scapula
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Plan of ossification of the scapula. Posterior side. Acromion visible at upper left, in blue.
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Radiograph of the shoulder showing an os acromiale
In other animals[edit]
The acromion process of bats (Mammalia: Chiroptera) is particularly elongated compared to that of humans.
Turtles have an acromion that forms the anterior part of the triradiate pectoral girdle (together with the coracoid and scapula). In this highly specialized endoskeletal structure, the scapula is a dorsal (directed upwards) process attached to the first rib; the coracoid is a posteroventral (directed backward and down) process; and the acromion is a medioventral (directed inwards and down) process (also known as the prescapular process) located at the base of the scapula. This had led to some controversy regarding the evolutionary origin of turtles, because in both pareiasaurs and non-mammalian therapsids the acromion is located at the dorsal tip of the scapula. [8]
In modern turtles, the acromion projects ventrally and articulates with the plastron (the flat lower part of the shell), but it evolved in a common ancestor of pareiasaurs and turtles long before the plastron. In these primitive ancestors, the acromion projected forward to form a strong and flexible articulation between the shoulder girdle and the clavicle. Notwithstanding these changes, the acromion of turtles retains its original function, to support the shoulder girdle and increase stride length. [9]
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The shoulder girdle of a snapping turtle. (1) Acromion, (2) scapula, and (3) coracoid
Additional images[edit]
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Right scapula. Acromion shown in red.
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Animation. Acromion shown in red.
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Right scapula. Posterior view. Acromion labeled at top left.
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Right scapula. Posterior view. Acromion shown in red.
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Right scapula. Posterior view. Acromion is "10"
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Right scapula. Anterior view. Acromion labeled at top right.
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Right scapula. Anterior view. Acromion shown in red.
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Right scapula. Anterior view. Acromion is "2"
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Left scapula. Lateral view. Acromion labeled at top right.
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Left scapula. Lateral view. Acromion is "4"
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Left scapula. Lateral view. Acromion shown in red.
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Anterior view. Acromion shown in red.
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Human arm bones diagram. Anterior view.
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Right human shoulder joint. Anterior view.
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The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula. Anterior view.
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Posterior view. Acromion shown in red.
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Muscles connecting the upper extremity to the vertebral column. Posterior view.
Notes[edit]
- ^ a b c Gray's Anatomy 1918, see infobox
- ^ Habermeyer, Magosch & Lichtenberg 2006, pp. 1–3
- ^ Warner, Beim & Higgins 1998, Introduction
- ^ Sammarco, VJ (March 2000). "Os acromiale: frequency, anatomy, and clinical implications". J Bone Joint Surg Am 82 (3): 394–400. PMID 10724231. Retrieved March 2, 2013.
- ^ Habermeyer, Magosch & Lichtenberg 2006, p. 4
- ^ Kurtz CA, Humble BJ, Rodosky MW, Sekiya JK. Symptomatic os acromiale. J Am Acad Orthop Surg 2006; 14:12-9.
- ^ Arend CF. Ultrasound of the Shoulder. Master Medical Books, 2013. Chapter on os acromiale available at ShoulderUS.com
- ^ Rieppel & Reisz 1999
- ^ Lee 1996, Abstract
This article incorporates text from a public domain edition of Gray's Anatomy.
References[edit]
- Habermeyer, Peter; Magosch, Petra; Lichtenberg, Sven (2006). Classifications and Scores of the Shoulder. Heidelberg: Springer. ISBN 978-3-540-24350-2.
- Lee, Michael S. Y. (January 22 1996). "The Homologies and Early Evolution of the Shoulder Girdle in Turtles". Proc. R. Soc. Lond. B 263 (1366): 111–117. doi:10.1098/rspb.1996.0018.
- Rieppel, Olivier; Reisz, Robert R. (1999). "The Origin and Early Evolution of Turtles". Annual Review of Ecology and Systematics 30: 1–22.
- Warner, Jon J.P.; Beim, Gloria M.; Higgins, Laurence (September 1998). "The Treatment of Symptomatic Os Acromiale". The Journal of Bone and Joint Surgery 80: 1320–6.
External links[edit]
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Bones of upper limbs (TA A02.4, GA 2.200–230)
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| Pectoral girdle, clavicle |
- conoid tubercle
- trapezoid line
- costal tuberosity
- subclavian groove
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| Scapula |
- fossae (subscapular, supraspinatous, infraspinatous)
- notches (suprascapular, great scapular)
- glenoid cavity
- tubercles (infraglenoid, supraglenoid)
- spine of scapula
- acromion
- coracoid process
- borders (superior, lateral/axillary, medial/vertebral)
- angles (superior, inferior, lateral)
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| Humerus |
- upper extremity: necks (anatomical, surgical)
- tubercles (greater, lesser)
- intertubercular sulcus
- body: radial sulcus
- deltoid tuberosity
- lower extremity: capitulum
- trochlea
- epicondyles (lateral, medial)
- supracondylar ridges (lateral, medial)
- fossae (radial, coronoid, olecranon)
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| Forearm |
- radius: upper extremity (head, tuberosity)
- body
- lower extremity (ulnar notch, styloid process)
- ulna: upper extremity (tuberosity, olecranon, coronoid process, radial notch, trochlear notch)
- body
- lower extremity (head, styloid process)
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| Hand |
- carpus: scaphoid
- lunate
- triquetral
- pisiform
- trapezium
- trapezoid
- capitate
- hamate (hamulus)
- metacarpus: 1st metacarpal
- 2nd
- 3rd
- 4th
- 5th
- phalanges of the hand: proximal
- intermediate
- distal
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anat (c/f/k/f, u, t/p, l)/phys/devp/cell
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noco/cong/tumr, sysi/epon, injr
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